The invention pertains to visual displays and more particularly to twisting-ball displays, such as gyricon displays and the like.
Gyricon displays, also known by other names such as electrical twisting-ball displays or rotary ball displays, were first developed over twenty years ago. See U.S. Pat. Nos. 4,126,854 and 4,143,103, incorporated by reference hereinabove.
An exemplary gyricon display 10 is shown in side view in FIG. 1A (PRIOR ART). Bichromal balls 1 are disposed in an elastomer substrate 2 that is swelled by a dielectric fluid creating cavities 3 in which the balls 1 are free to rotate. The balls 1 are electrically dipolar in the presence of the fluid and so are subject to rotation upon application of an electric field, as by matrix-addressable electrodes 4a, 4b. The electrode 4a closest to upper surface 5 is preferably transparent. An observer at I sees an image formed by the black and white pattern of the balls 1 as rotated to expose their black or white faces (hemispheres) to the upper surface 5 of substrate 2.
A single one of bichromal balls 1, with black and white hemispheres 1a and 1b, is shown in FIG. 1B (PRIOR ART).
Gyricon displays have numerous advantages over conventional electrically addressable visual displays, such as LCD and CRT displays. In particular, they are suitable for viewing in ambient light, retain an image indefinitely in the absence of an applied electric field, and can be made lightweight, flexible, foldable, and with many other familiar and useful characteristics of ordinary writing paper. Thus, at least in principle, they are suitable both for display applications and for so-called electric paper or interactive paper applications, in which they serve as an electrically addressable, reuseable (and thus environmentally friendly) substitute for ordinary paper. For further advantages of the gyricon, see U.S. Pat. No. 5,389,945, incorporated by reference hereinabove.
Known gyricon displays employ spherical particles (e.g., bichromal balls) as their fundamental display elements. There are good reasons for using spherical particles. In particular:
Spherical bichromal balls can be readily manufactured by a number of techniques. See the ""098 and ""594 patents, incorporated by reference hereinabove, in this regard.
Spheres are symmetrical in three dimensions. This means that fabrication of a gyricon display sheet from spherical particles is straightforward. It is only necessary to disperse the balls throughout an elastomer substrate, which is then swelled with dielectric fluid to form spherical cavities around the balls. The spherical balls can be placed anywhere within the substrate, and at any orientation with respect to each other and with respect to the substrate surface. There is no need to align the balls with one another or with the substrate surface. Once in place, a ball is free to rotate about any axis within its cavity.
xe2x80x9cIn the xe2x80x98whitexe2x80x99 state, the gyricon display reflects almost entirely from the topmost layer of bichromal balls and, more particularly, from the white hemispherical upper surfaces of the topmost layer of balls. In a preferred embodiment, the inventive display is constructed with a single close-packed monolayer of bichromal balls.xe2x80x9d
Ideally, a close-packing arrangement would entirely cover the plane with the monolayer of gyricon elements. However,. Inasmuch as a planar array of spheres cannot fully cover the plane, but must necessarily contain interstices, the best that can be achieved with a single population of uniform-diameter spherical elements is about 90.7 percent areal coverage, which is obtained with a hexagonal packing geometry. A second population of smaller balls can be added to fill in the gaps somewhat, but this complicates display fabrication and results in a tradeoff between light losses due to unfilled interstices and light losses due to absorption by the black hemispheres of the smaller interstitial balls.
Therefore, it would be desirable to provide a close-packed monolayer gyricon display in which areal coverage surpasses 90.7 percent or approaches 100 percent, without any need for interstitial particles. This can be done by using cylindrical rather than spherical elements. For example, a rectangular planar monolayer array of cylinders can be constructed that entirely or almost entirely covers the plane. With the white faces of the cylinders exposed to an observer, little if any light can get through the layer.
The invention provides a gyricon display having cylindrical, rather than spherical, rotating elements. The elements can be bichromal or polychromal cylinders, preferably aligned parallel to one another and packed close together in a monolayer. The close-packed monolayer configuration provides excellent brightness characteristics and relative ease of manufacture as compared with certain other high-brightness gyricon displays. The cylinders can be fabricated by techniques that will be disclosed. The substrate containing the cylinders can be fabricated with the swelled-elastomer techniques known from spherical-particle gyricon displays, with a simple agitation process step being used to align the cylinders within the sheeting material.
Further, the invention is well-suited to providing a gyricon display having superior reflectance characteristics comparing favorably with those of white paper. A gyricon display is made with a close-packed monolayer of cylinders, wherein cylinders are placed, preferably in a rectangular packing arrangement, so that the surfaces of adjacent cylinders are as close to one another as possible. The light reflected from the inventive gyricon display is reflected substantially entirely from the monolayer of cylinders, so that lower layers are not needed. The areal coverage fraction obtainable with cylinders is greater than that obtainable with a single monolayer of uniform-diameter spheres.
In one aspect, the invention provides a material comprising a substrate and a plurality of nonspheroidal (e.g., substantially cylindrical) optically anisotropic particles disposed in the substrate. A rotatable disposition of each particle is achievable while the particle is thus disposed in the substrate; for example, the particles can already be rotatable in the substrate, or can be rendered rotatable in the substrate by a nondestructive operation performed on the substrate. In particular, the substrate can be made up of an elastomer that is expanded by application of a fluid thereto so as to render the particles rotatable therein. A particle, when in its rotatable disposition, is not attached to the substrate. A display apparatus can be constructed from a piece of the material together with means (such as an electrode assembly) for facilitating a rotation of at least one particle rotatably disposed in the substrate of the piece of material.
In another aspect, the invention provides a material comprising a substrate having a surface and a plurality of nonspheroidal optically anisotropic particles disposed in the substrate substantially in a single layer. The particles (e.g., cylinders) are of a substantially uniform size characterized by a linear dimension d (e.g., diameter). Each particle has a center point, and each pair of nearest neighboring particles in the layer is characterized by an average distance D therebetween, the distance D being measured between particle center points. A rotatable disposition of each particle is achievable while the particle is thus disposed in the substrate. A particle, when in its rotatable disposition, is not attached to the substrate. Particles are sufficiently closely packed with respect to one another in the layer such that the ratio of the union of the projected areas of the particles to the area of the substrate surface exceeds the areal coverage fraction that would be obtained from a comparably situated layer of spheres of diameter d disposed in a hexagonal packing arrangement with an average distance D therebetween as measured between sphere centers. If the ratio D/d is made as close to 1.0 as practicable, the ratio of the union of the projected areas of the particles to the area of the substrate surface can be made to exceed the maximum theoretically possible areal coverage fraction for a maximally close-packed hexagonal packing geometry of a layer of spheres of diameter d, which is approximately equal to 90.7 percent.
The invention will be better understood with reference to the following description and accompanying drawings, in which like reference numerals denote like elements.